This is a U.S. original application which claims priority on French patent application No. 0108833 filed Jul. 4, 2001.
The present invention relates to a process for grinding a knife shaft and the control device linked to the implementation of the process. The knife shaft is used in a machine intended for cutting sheets of material into strips, for example, sheets of paper, plastic, plates of photosensitive film or any other material having the form of thin sheets.
In the photographic industry, to obtain several strips of photosensitive film from an initial strip of large width, slitters are used comprising many rotary knives mounted in spaced apart manner on a first knife shaft, and many counter-knives mounted on a second knife shaft, with the strip to be cut running between these two rows of knives and counter-knives. In place of knife shafts, independent units can be used carrying the knives or counter-knives. It is necessary that the knives and counter-knives be sharpened regularly to maintain a good quality of cut on the edge of the cut strips.
There already exist many means that enable the taking into account of the sharpening done on the knives of various slitters, by compensating dimensionally using appropriate means, for the loss of material due to the sharpening of one or more knives. These compensation means enable sufficiently good control of the cutting process to be kept over time, following successive sharpening of the knives. This control of the cutting process produces a sufficiently good cutting quality of the cut strips and little dimensional variability of these cut strips. However, this dimensional variability remains excessive compared with the specifications of film strips used in the photographic industry.
U.S. Pat. No. 4,592,259 describes a method and means for adjusting the relative positioning of the slitter knives of a strip cutting apparatus; in order to obtain a correct relative position of the knives one with another, and between each of the cutting units taking these knives; the cutting units can move on slides. Electrical and mechanical means enable automatic compensation for the dimensional variations of thickness of the knives in time. These compensations produce adjustments of the position of the cutting units one with another on their slides. The objective is to obtain a constant and specified distance between the cutting edges of two successive knives, by comparison with a standard reference value recorded in a memory, and corresponding, for example, to the thickness of a new blade. This invention enables a constant distance between the knives to be obtained, but this concerns knives belonging to slitters or carriages that are independent one from another as to their relative movements on their respective slides. In other words, the overall geometry of the cutting means modifies according to the dimensional variations of the knives, to keep constant the distance between the cutting units and therefore between the cut edges of the knives.
U.S. Pat. No. 4,607,552 describes an apparatus enabling automatic control of the position of many slitters that cut a moving strip. Electronic control means enable, from the measurement of wear of the cutting blades of each slitter, calculation of the dimensional compensation to correctly reposition the blade, relative to the strip to be cut and to the part acting as the counter-knife. This apparatus thus enables compensation of the wear of each of the slitter""s blades, independently one from another.
The object of the invention disclosed in U.S. Pat. No. 5,097,732 has certain similarities with that of U.S. Pat. No. 4,607,552. A numerical control device enables the measurement and control of the interval between the cutting units of a slitter having many cutting units. The objective of the invention is to be able to move many cutting units simultaneously to a preset position. Then after this movement of the cutting units, the respective adjustment of the contact pressures of the upper and lower knives is carried out.
U.S. Pat. No. 4,072,887 discloses an apparatus enabling the movement of mobile elements, especially a first pair of circular cutting blades working together having their axes parallel, into a new position, through a translation according to the axis of the circular cutting blades. The apparatus enables the repositioning, using appropriate measuring means, of successive pairs of blades located side by side on independent units, compared with the first pair of blades moved.
European Patent Application 0,602,655 describes a sharpening method for circular cutting blades attached to a shaft. This invention especially aims to not remove the blades from the same knife shaft to sharpen them and so avoid inducing causes of error and thus dimensional variations linked to the remounting operation of these blades on their shaft after their sharpening. The sharpening operation described in this invention especially enables, from the knife shaft comprising its blades to be sharpened and mounted between points on a grinder, to plunge one or more rotating grinding wheels towards the edges of the blades by ensuring the movement of the grinding wheel with a numerically controlled programmed device. This is in order to sharpen successively or simultaneously the cutting blades of the same shaft without removing the blades. The final objective being to improve the lateral and radial run-out of the blade cutting edges by increasing the precision obtained on the cut strips of product. However, the result obtained as to the strip widths of product cut with the knife shafts sharpened according to this sharpening method remains unsatisfactory.
French Patent Application 9912181 relates to a device and a process to position many knives mounted on a first knife shaft in relation to many counter-knives mounted on a second knife shaft of the same strip slitter. This does not enable ensuring especially the dimensional constancy or reproducibility of the pitch on a given slitter.
All the means described in the above mentioned documents are based on principles and means of control or measurement enabling the positioning or repositioning one against the other, of cutting units or slitters comprising knives, to compensate for example for the parameters of variability of the cutting process. The purpose of this is to conserve overall control of the process. In the case of slitters, an important variability parameter of the known process is the wear of the knife blades used on these machines. This phenomenon can be controlled by acting on certain physical components of the slitter, for example, by moving them one in relation to the others to compensate for example for the wear of the knives. It is possible on the same slitter to change, for example, the type of manufacture and proceed to remove the knives corresponding to a first type of manufacture to replace them by other knives corresponding to a new planned manufacture. Then later, for example, all or part of the knives corresponding to the first type of manufacture may be reused. In this case, appropriate control and measuring means enable the control and repositioning if necessary of the knives one in relation to the others; but the guarantee of the reproducibility of the axial pitch between the knives is not assured when sharpening; consequently the quality of the cut obtained by a good correspondence or good pairing of the respective knives of the two knife shafts working together to cut, for example, the same strip of material is not assured. In other words, the means used in the prior art mentioned enable control of the cutting process but without controlling the reproducibility or the variability of the cutting pitch of the knife shaft.
An object of the present invention is to control the evenness of sharpening the knife shafts of the same slitter, and more precisely pairs of knife shafts equipped with knives, so that over time and with successive sharpening or grinding, these knife shafts, for a specified cutting width, have a pitch between their respective knives that is perfectly controlled and even along with the grinding; which guarantees good pairing of the two shafts. Thus advantageously special additional adjustments of one shaft in relation to the other on the slitter taking these two shafts are prevented; all without generating dimensional drift or scatter of the various cutting pitches in time. The present invention enables a robust grinding process to be obtained and maintained, while making productivity gains, as the grinding of the knife shafts is done in concurrent time on a special grinding machine. For a given pair of knife shafts, initial adjustments of the slitter are no longer necessary, as the two paired knife shafts of the same slitter will have knives that stay well positioned one in relation to the other, during successive grinding. Thus what is obtained is not only excellent mastery of the precision of the specified cut width, but also and above all a better cut due to at least the control of the variability of the axial pitch between the various knives; this enables dimensional evenness of the knife shafts to be obtained along with the grinding. It is even possible to contemplate interchangeability between the knife shafts of different pairs of knife shafts, given the precision level and low dimensional variability obtained with the process according to the invention.
One advantage of the process according to the invention is that it is independent of the variability parameters, e.g. mechanical, due to the grinding machine.
Another advantage of the process according to the invention is that it enables keeping good geometric positioning of the knives and a constant pitch independently from variations of the physical parameters linked to the grinding machine""s environment. One of these parameters is, for example, the ambient temperature.
The usefulness of this process is precisely being able to correct each knife of a knife shaft by evening up the dimensions of the individual pitches between two consecutive knives without depending on the variability of the grinding machine""s mechanical components.
The present invention relates to a grinding process of many knives placed on the periphery of a knife shaft, a process characterized by the following steps:
a) define the difference of the actual position of each knife in relation to a reference position corresponding to the theoretical positions of the knives, by determining for each different pair of consecutive knives of the knife shaft the algebraic value of the difference between the actual pitch measured between two consecutive knives and the theoretical pitch;
b) calculate the average algebraic value of the algebraic values of the differences between the actual pitch and the theoretical pitch determined at step a), by dividing the sum of said algebraic values of the differences by the total number of different pairs of consecutive knives of the knife shaft;
c) determine the algebraic value corresponding to a first corrected relative position of each knife, by removing said average algebraic value of the differences calculated at step b) from each of the algebraic values of the difference between the actual pitch and the theoretical pitch determined at step a);
d) determine the algebraic value of the difference between the total actual length between the two end knives of the knife shaft, and the total theoretical length of the knife shaft calculated by multiplying the total number of different pairs of consecutive knives by the value of the theoretical pitch;
e) determine the algebraic value of the difference for the length per knife by dividing the algebraic value of the difference between the total theoretical length and the total actual length obtained at step d) by the total number of knife pairs of the knife shaft;
f) determine the algebraic value corresponding to a second corrected relative position of the knives by adding the algebraic value of the difference for the length per knife to the algebraic values corresponding to the first corrected relative position; and
g) from the sum of the algebraic values of the second corrected relative position, determine the quantities of material to be removed per knife.
Other characteristics will appear on reading the following description, with reference to the drawings.